In today's commerce, companies and organisations often have departments at widely separated geographical locations. Each geographical location is usually provided with a private branch exchange, PBX. The PBX connects telephones at that site to a number of outgoing lines connected to a public network or private circuit routes. The private circuit routes allow employees of the company to communicate with employees at other sites and to use call services as desired. It has been found desirable for personnel in the different departments to have available to them a range of call services on their telephone networks. These services may include "call-back-when-free", "call-back-when-next-used", "divert-on-busy" and "centralised-operator".
The call service "call-back-when-free" is a service which caters for the situation where a first telephone user wished to contact a second telephone user but is unable to do so because the second telephone user is engaged on another call. The first telephone user dials a code number for the "call-back-when-free" call service and replaces his handset. This causes his telephone to ring when the second telephone user finishes his present call and replaces his handset. By lifting his handset, the first telephone user can then cause the second user's telephone to ring and the call can then be made.
The call service "call-back-when-next-used" is a call service which caters for the situation where the first telephone user attempts to contact the second user but although a ringing tone is heard the second user does not answer the telephone. The first user then enters a code for this service and replaces his handset. This causes a "registration" of the request for call-back-when-next-used at the PBX serving the second user's telephone. The PBX monitors the second user's telephone for activity. When a call is made by the second user and then cleared from his telephone, the PBX notifies the PBX serving the first user's telephone. The PBX serving the first user's telephone then sets up a "no-ring" call which establishes a speech path between the two telephones. Only the first person's telephone is made to ring. When the first user answers his telephone, the second user's telephone is made to ring and the call is established.
The call service "divert-on-busy" caters for the situation where the first telephone user wishes to contact the second telephone user but that user is engaged. The second telephone user may have programmed his telephone system to divert calls to a colleague's extension when he is engaged on a call.
The call service "centralised-operator" allows one telephone extension of a local telephone network to act as the operator on behalf of other extensions of the entire private telephone network.
FIG. 1 shows a conventional communications network topology in which the network comprises four private branch exchanges 1 to 4, a public switched telephony network 5 and communication paths 6 to 12.
The private branch exchanges (PBX) 1 to 4 serve local telephone networks at various sites of the same company. It will be readily appreciated from the figure that there are a number of possible routes for calls routed from one local telephone network to another along communication paths 6 to 12.
For example, suppose a call originating from a local telephone network served by PBX2 is to be routed to a local telephone network served by PBX3. It may be routed via communication path 8, the public switched network 5 and communication path 10 to arrive at PBX3. Alternatively, the call may be routed via communication path 9 to arrive at PBX3. This route is called a private circuit because it does not involve the call being carried over the public switched network 5.
In a similar manner, calls may be routed from PBX4 to PBX2 by means of communication path 11, the public switched network 5, and communication path 8. Alternatively, a private circuit may be used, comprising communication path 12, PBX3 and communication path 9, to arrive at PBX2.
Current PBX designs accept calls from an associated network of telephones at that site governed by a first communication protocol and pass that call to the public network by means of a second protocol. An example of the first protocol is digital private network signalling system (DPNSS) and an example of the second is digital access signalling system No2 (DASS2). DPNSS is a protocol which has been designed to support call services but the DASS 2 protocol does not support call services.
Private circuit routes allow the PBXs to communicate using the first protocol DPNSS and hence allow supplementary services. For geographically distant sites, it is usual for the organisation to rent a dedicated communication path from a telephone service provider. In this case the dedicated communication path is path 9. However, generally speaking, these paths are often underutilised and this is especially the case for primary rate 2.048 Mbit/s paths. This results in the company or organisation having to pay for dedicated communication paths which can carry a greater volume of call traffic than the company generates.
International Patent Application WO93/15583 discloses a communications network arrangement comprising a first communications network ("public network") which operates in accordance with a first signalling protocol, and a plurality of further communications networks ("PABXs") which are connected to the first communications network and which belong to the same virtual private network. The further communications networks operate in accordance with a second signalling protocol (which may be different for different ones of the further communications networks) which supports a call service which is not supported by the first protocol. Each PABX is provided with an "interworking unit" which sits longside the PABX providing the virtual network to the PABX. All private network calls route through the interworking unit which uses a database to translate the incoming private network number to a public network number and establishes an outgoing call to that public network address across another private interface. This is routed back through the PABX so that the public network interfaces that are required on the PABX can be used. The address provided on the secondary call identifies the call, but need not route the call to the final extension across the public network. The signalling necessary to establish the final part of the call on the other side of the network is sent through an overlay network, for example an X.25 network. Calls are established within the X.25 network as required to support the private network. Thus dedicated private circuit functionality is provided using separate signalling and transmission network bearers.